Relative Humidity and How It Influences Moisture Content

How Relative Humidity (RH) Affects Wood Moisture Content (MC)

If you work with wood, you need to understand how it interacts with, and is affected by, the relative humidity (RH) of the environment. This is true if you install fine hardwood floors, make furniture like cabinets and tables, or if you use lumber for construction.

You need to understand how the RH affects wood for one simple reason.

Wood is hygroscopic.

This is just a fancy way of saying that wood gains or loses moisture in response to changes in the RH. If wood has less moisture than the surrounding air, it will absorb moisture from the air. If wood has more moisture than the surrounding air, it will release moisture into the air.

Eventually, after wood has been in any particular environment for a period of time it will reach a point where it’s no longer absorbing or releasing moisture. It will be in balance with its environment. We call this balanced state the Equilibrium Moisture Content or just ‘’EMC’’ for short.

So, why exactly is it important to understand this property of wood?

Well, it’s important to understand because of what happens to the wood as it’s absorbing and releasing moisture…

It changes physical size.

Wood physically shrinks or expands as it loses or gains moisture.

As the RH increases, wood’s moisture content (MC) increases, and because of that, its physical size increases. It expands. As the RH goes down though, wood’s MC decreases, and its physical size decreases. It shrinks.

And therein lies the problem, which we’ll get to in a bit.

So, when wood has reached the EMC, it’s no longer shrinking or expanding. As we noted above, wood will attain the EMC after it has been in a location for a period of time. That could be weeks or even months in some cases.

This is true if the wood is lumber in the manufacturing process, an unfinished product in transit or storage, or a finished wood product.
Ok, so now let’s talk some more about what happens when wood shrinks or expands in size. Specifically, what it means for your wood projects.

How to Avoid Moisture-Related Deformities in Your Wood Projects

Nobody wants to spend time creating a valuable wood project — of any kind — only to see it later ruined by moisture-related problems. However, that could happen if you don’t understand how RH influences the moisture levels of wood and what happens to wood’s physical dimensions as it absorbs and releases moisture. So, let’s talk more about that now.

A 4% change in wood MC will cause wood to either shrink or expand about 1% across the grain. This does vary according to species. However, we can safely say that it’s around 1%. In contrast, even with large changes in MC, wood shrinks or expands very little along the grain.

Now, 1% might not sound like a lot. However, even physical changes this small can cause serious problems. Problems with gluing, for example. This can happen with cabinets, tables, flooring, or any other wood project. Drawers that don’t fit right, problems with doors, joints, etc. You might even see warped and cracked finishes.

To give you an example, here’s what can happen with wood flooring.

A 1/32” change per 2” multiplied across an 8-foot room turns into 1-½”. That will be hard to miss.

So, it’s a small change, but it has a big effect.

The most important point to understand is that if wood isn’t at the EMC where the project will be finally used, it will continue to either shrink or expand afterward. This is how deformities happen.

However, this is an easy problem to avoid if you know how to measure the MC of wood.

Make Sure You Keep Wood at the Desired MC

When wood is already at its desired MC, you need to use a quality moisture meter to make sure the wood doesn’t continue to absorb or release moisture. Why? Well, it’s time-consuming to get wood back to its desired MC and as we’ve just said you don’t want to create wood projects using wood that’s not at the proper MC.

Temperature Has No Significant Effect on Wood MC

Many people assume that temperature factors into this equation somehow. Actually, it doesn’t. Temperature does not appreciably affect wood’s MC at all. To put it another way, an increase or decrease in ambient temperature will not cause any significant shrinkage or swelling in wood. This lack of temperature sensitivity is one of wood’s advantages when compared to other building materials such as aluminum or plastic.

Understanding Average RH, Average Wood MC, and EMC

For any given average RH there’s an average MC to which wood will equilibrate. We can see this in the following chart. If you work with wood, you might want to memorize this information:

RH of the in-use locationEMC of the in-use locationCorresponding MC the wood will attain at this location
19-25%5%5%
26-32%6%6%
33-39%7%7%
40-46%8%8%
47-52%9%9%

For example, if the RH in the air is 30%, wood stored in this environment will eventually reach a 6% MC. We can also see from this chart that if the RH in the air is 50%, wood will eventually reach an MC of 9%. Now, this might take weeks or even months. It all depends on the type of wood, the size of the wood, the grain orientation, the ambient temperature, and if any coating has been applied to the wood.

Note that the MC wood would eventually attain if it’s stored in a 30% or 50% humidity environment is the same as the EMC. That is, 6% or 9%.

These two figures are important to remember because they’re the typical interior values (at typical room temperatures) for heated and air-conditioned homes and offices in most of North America.

Of course, in cold winter conditions, the interior conditions might be drier than 6% EMC. In fact, in manufacturing facilities that have either been fitted with exhaust systems for dust or heated for comfort, the conditions are usually drier than 6% EMC unless they’re being humidified.
In humid summer months, the opposite is true. The EMC in an interior without air conditioning may exceed 9%.

For exterior structures such as sheds, warehouses, or homes under construction without heat, the RH might be 65% with a 12% MC and EMC.
For hot, very humid southern climates like Florida and the Carribean, the RH will be around 80% with a 16% EMC.

Note: You might be tempted to think that this exchange of moisture between wood and its environment doesn’t happen if the wood is coated. Not true. Coated wood goes through the same changes in response to RH increases and decreases. It just takes longer to reach its EMC than uncoated wood.

So, what does all this mean for those who work with wood?

It means that wood should always be allowed to come into balance with the environment where it will be finally used. In other words, it must be allowed to reach the EMC before you start working with it, whether that means putting down a hardwood floor, making a cabinet, or building some other kind of wood product. Failure to do this could result in post-construction problems such as cracking or warping.

Something else…

The MC of Kiln-Dried Wood Is Not Fixed

Some people assume that once wood is kiln-dried down to a certain MC, the MC is fixed. This is incorrect. Kiln-drying does not give us wood that’s unresponsive to RH changes. All it does is quickly dry wood down to the desired MC. If the kiln-dried wood isn’t handled properly, it will regain moisture.

Understanding how wood is affected by RH is necessary if you want your projects to be a success. Once your wood is at its desired MC, you should then use a quality moisture meter to ensure that it stays that way.

Our Orion® series of pinless wood moisture meters can help you do this. They’re extremely accurate and come with an On-Demand Calibrator that allows you to recalibrate the meter on-site to NIST-traceable standards in about a minute. This easy-to-use feature makes it possible for you to calibrate the meter without sending it back to the manufacturer. Just select the feature, push a button and place the meter on the calibrator. Other moisture meters only tell you how far out of calibration the meter is. They can’t recalibrate. For that, you need to send the meter back to the manufacturer.

The Orion® 910, 930, 940, and 950 meters also come with IntelliSense™ technology that enables the meter to “overlook” excess moisture on the surface of the wood.

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